In the nuclear industry, metal equipment such as pumps, valves, piping, ventilation ductwork, gloveboxes, machinery, tooling, and structural members may become surface-contaminated with radioactive substances. This surface contamination may be from actinides such as plutonium or americium, fission products as encountered in nuclear fuel reprocessing, or radioactive deposits in nuclear reactor piping.
When such contaminated equipment is taken out of service, it must be decontaminated before being either returned to service or otherwise disposed of. As radioactively-contaminated waste may require expensive continuing care, it is desirable to minimize the volume of material requiring such care. Furthermore, as the contaminated metal or the contaminant may be valuable, it could be desirable to recover as much as possible.
In the disposal of metal articles surface-contaminated with plutonium, those items showing a radioactivity of greater than 23,454 alpha disintegrations per minute per square inch (d/m/in.sup.2) are placed in "retrievable" storage. This is expensive storage with the article being hermetically contained and available for further processing at a future date. Articles having radioactivity less than 23,454 alpha d/m/in.sup.2 are classified as "non-retrievable" waste and are eligible for permanent burial in geological strata. This disposal option is relatively expensive. Articles having radioactivity of less than 32 alpha d/m/in.sup.2 may be disposed of in relatively cheap "landfill" disposal. Articles having a surface radioactivity of less than 1 alpha d/m/in.sup.2 are essentially "background" and require no special treatment or disposal.
Prior art techniques for removing surface contamination from metal articles include such manual techniques as washing, scrubbing, and mechanically abrading the surface of the metal article. These techniques are generally successful in removing gross deposits of contamination from exterior surfaces but are unsuccessful in removing contamination from interior surfaces, cracks, crevices, or pores in the surface of metal articles. The metal waste would then be stored in one of the expensive permanent care facilities.
A recent development in the decontamination of metal is electropolishing which comprises electrolytically dissolving a surface layer of the metal article in a bath of phosphoric acid. The plutonium which is thereby removed from the surface of the metal is then recovered from the phosphoric acid solution by sophisticated chemical techniques such as ion exchange, solvent extraction, chemical precipitation, distillation, electrolysis, or membrane separation. The spent phosphoric acid solution is then neutralized and itself be disposed of as radioactive waste. When handling fissile material, criticality safety requires a nuclear poison. This can be achieved by the addition of cadmium to the phosphoric acid electrolyte but the cadmium would then be an additional disposal problem because of its health hazard.